It seems like MailChimp uses a URL encoded POST to add contacts to their API. When adding single contacts through our site, we require a POST of Atom XML. If you are using our PHP wrapper, you may not need to do much modification to what you already have. Or you could just take the example in the code sample section and modify it to what you need.
You can find the code samples for PHP here, and a new PHP wrapper that we released recently here, including a wiki.